Plate tectonics different on early Earth?

A paper published this week in Science uses diamonds to paint a new picture of …

Plate tectonics is the great unifying theory of geology, which makes it all the more amazing that it has only been accepted for about 50 years. If you think we’ve got it all figured out by now, a paper published this week in Science may surprise you. And you'd be wrong if you were expecting to read about some dusty rock cores. The new information comes from a much shinier source: diamonds.

Contrary to popular culture, diamonds are not formed from the metamorphosis of coal under tremendous heat and pressure. It makes for nice poetry, but it’s not true. The real story is actually a bit more interesting than that.

Diamonds form in the upper mantle, about 125 to 175 km below the surface, where they form along the undersides (or “keels”) of continents. After they crystallize, they commonly spend more than a billion years before coming to the surface in spectacular fashion. Author Bill Bryson describes that process best in A Short History of Nearly Everything: "What happens is that deep in the Earth there is an explosion that fires, in effect, a cannonball of magma to the surface at supersonic speeds." Poetry in motion, if you will.

During the long, slow crystallization, little bits of other minerals get encapsulated in the growing diamond. As the authors of the new paper point out, those little mineral bits are apparently "the oldest, deepest, and most pristine samples of Earth’s mantle," making them a unique source of data on the history of plate tectonics. There are a couple different radioactive decay series that allow geochemists to determine the age of these diamonds, and the chemistry of the mineral inclusions tells researchers whether the diamond formed in an area where tectonic plates were colliding, such as at a subduction zone.

The ultimate result of plate tectonics is something called the Wilson Cycle, which goes something like this: hot, upwelling material creates a spreading center, which opens a new ocean basin bounded by subduction zones. In time, the spreading ceases, the basin closes up, and the two continents collide, building a mountain range.

The authors of this study used diamonds to see how far back in Earth history the Wilson Cycle has been operating. They compiled a data set from past studies, with about 4,400 diamond inclusions from the ancient cores of five continents. While they found diamonds that formed in normal mantle conditions (that is, not near tectonic collisions) up to 3.5 billion years old, the diamonds with inclusions from subduction zones maxed out at about 3.0 billion years.

The researchers say this indicates that proper Wilson style plate tectonics did not occur until 3 billion years ago, about 1.5 billion years after the Earth first formed. They cite other work that has dated the oldest indications of subduction at around 3 billion years as well.

So what does this mean about the early Earth? The authors suggest that, at the earliest points, "continental nuclei formed by non-Wilson cycle processes." There’s still evidence for some recycling of crustal rock into the mantle at that time, so the researchers expect that other, much different processes dominated the scene.

They suggest that the mineral chemistry of the early mantle was so different that oceanic crust was simply too buoyant to be subducted deep into the mantle, leading to very shallow plate recycling. Local downwelling in mantle circulation may have acted to drag plates down just far enough for some melting to take place. Perhaps simple hydrothermal alteration played a significant role in the chemical exchange between the mantle and crust. In short: we don’t really know, but it would have looked very different from today.

The geologic conditions of the early Earth are still a source of tremendous debate, and this paper is likely to fan the flames. This is definitely not the final word, but it makes a pretty strong case that picturing the infancy of our planet will require an even greater stretch of the imagination than we previously realized.